A siamese cable comes from the box at the end of the street, one for the phone line and one for the broadband/tv services. Your question seems a little confusing as to whether they supply their phone lines through the cable or through the telephone line, I don't understand what you mean? If you mean do they use the BT line then no they don't, it's separate. Also since it is a shared house you can only have one Virgin Media account per household so if you already have an account then another person in your house won't be able to have one
Hope this helps